For Melissa McCarthy, it’s always the “Bridesmaid.” Ever since hitting it big in 2011’s biggest comedy, the Oscar-nominee has pretty much played variations on Megan, the crass-and-earnest schlub who made you afraid to ever enter a bridal boutique again. We saw subsequent facsimiles of her in “This is 40,” “The Heat” and “Identity Thief.” If she doesn’t stray from this path of wasted talent, she’ll be Adam Sandler before you know it.

With “Tammy,” a road-trip picture that never veers far from the Illinois neighborhood the title character flees after being fired at the fast-food joint Topper Jacks and discovering her husband (Nat Faxon) doing the horizontal boogie with their neighbor (Toni Collette), it’s pretty much a series of uninspired and cliched pratfalls that rely on McCarthy’s plus-sized physique.

It’s embarrassing. And considering she co-wrote the script with her husband, Ben Falcone, McCarthy has no one to blame but herself. In working with Falcone, who also makes his directorial debut, the duo struggle to veer from the usual array of laugh-starved sit-com antics. McCarthy can hold her weight with any physical comedian, but she’s too talented to be reduced to always playing the heavy girl who stuffs pies in her mouth or falls asleep with a package of powdered doughnuts in her hand. From the moment we meet her, Tammy is a disheveled mess bearing an orange-blonde dye job and dirty Crocs. She drives a green Toyota jalopy and sings ‘80s songs: The Outfield’s “Your Love.”

The scene where Tammy gets fired by her boss (Falcone) is a funny showcase for McCarthy’s rude-and-crude side. But it’s all downhill from there, as she hits the road for Niagara Falls with Grandma Pearl, a randy lush played by Susan Sarandon, who is a long way from “Thelma and Louise.” The usual road-trip tropes of bonding and redemption predictably occur, but first the two must endure a gauntlet of misadventures involving booze, cops, robbery and a lesbian Fourth of July party. A stacked supporting cast with Mark Duplass, Kathy Bates, Gary Cole, Sandra Oh and Allison Janney doesn’t forgive the dumb, predictable story that we’ve seen a hundred times before.